(1) Field of the Invention
This invention relates to a solar simulator, and more particularly to a solar simulator which enjoys simplicity of construction and approximation to the spectral (energy) distribution of natural sunlight and permits optical intensity to be varied without affecting spectral (energy) distribution.
(2) Description of the Prior Art
The solar simulator, as well known, is a light-source device for reproducing the spectral distribution of natural sunlight with high accuracy. This solar simulator is indispensable to determination of performances of various devices using solar energy, such as the photoelectric conversion property of a solar cell, and to accelerated degradation test of such performances.
In the conventional solar simulators, xenon short arc lamps have been popularly used. Incidentally, the light from the xenon short arc lamp has a group of sharp and complicate peaks in the near infrared region (800 to 1,000 nm) as illustrated in the spectral distribution diagram of FIG. 1. Frequently, therefore, the xenon short arc lamp is used in combination with a multilayer interference filter adapted to compensate these peaks evenly and approximate the spectral distribution of this light to that of natural sunlight.
A typical spectral distribution of the light emitted from the solar simulator using the xenon short arc lamp so compensated as described above is indicated by the solid line in FIG. 2. The chain line found in the diagram indicates the spectral distribution of natural sunlight (under the condition of air mass zero).
As noted also from FIG. 2, in the conventional solar simulator, the spectral distribution thereof is fairly close on the average to that of natural sunlight and the conventional solar simulator can be put to practical use.
For more accurate measurement of the photoelectric conversion property exhibited under natural sunlight by the solar cell of varying grade possessed of spectral sensitivity characteristics over a wide range from ultraviolet region through near infrared region as illustrated in FIG. 3, the conventional solar simulator which resorts to the combination of a xenon lamp and a multilayer interference filter or, generally a dichroic filter is still insufficient.
This is because a small group of peaks remain in the near infrared region (in the range of 750 nm) to 1,000 nm as noted from the spectral distribution diagram of FIG. 2 and these peaks cause an error in the measurement.
As means of diminishing the aforementioned group of peaks thereby approximating the spectral distribution to that of natural sunlight and enhancing the spectral accuracy, it has been proposed to combine the light emitted by the xenon short arc lamp having a relatively continuous spectral distribution in the ultraviolet region through the visible region and the light emitted by the incandescent filament (tungsten halogen) lamp having a continuous spectral distribution in the near infrared region in an overlapped or mixed state.
A typical synthetic spectral distribution obtained by the combination is illustrated in FIG. 4.
In the diagram, the curve L1 represents the spectral distribution characteristic of the light emitted by the xenon short arc lamp minus the component falling on the longer wavelength side than the near infrared and the curve L2 the spectral distribution characteristic of the light emitted by the incandescent filament lamp minus the components of visible light and ultraviolet light.
The curve L3 represents the synthetic spectral distribution characteristic obtained by having the aforementioned curves L1 and L2 combined in an overlapped or mixed state. The curve L4 of solid line representing the same spectral distribution characteristic of natural sunlight as shown in FIG. 2 for the purpose of comparison.
It is seen from FIG. 4 that a spectral distribution (curve L3) satisfactorily approximating the spectral distribution (curve L4) of natural sunlight is obtained and the group of irregular peaks in the near infrared region which has been responsible for the erroneous measurement obtained by the conventional solar simulator can be diminished by combining the light emitted by the xenon short arc lamp minus the component on the longer wavelength side than the near infrared light (curve L1) and the light emitted by the incandescent filament lamp minus the components of visible light and ultraviolet light (curve L2) in an overlapped or mixed state.
In a generally conceivable specific configuration of the solar simulator possessing such a spectral distribution as described above, a first light-source device combining a xenon short arc lamp and a filter capable of eliminating the light on the longer wavelength side than the near infrared light and a second light-source device combining an incandescent filament lamp and a filter capable of eliminating the light in the visible and ultraviolet regions are prepared and disposed in such a manner that the beams of light emitted from these two light-source devices will be directed to a single integrating optical system to be combined in an overlapped or mixed state.
It is suspected, however, that this configuration entails the following drawbacks.
(1) Since the configuration necessitates use of the same number of filters (such as dichroic filters) as the total number of xenon short arc lamps and incandescent filament lamps, the apparatus used therefor is large and the maintenance of this apparatus is complicated and the cost of the apparatus is high.
(2) It is extremely difficult for the plurality of filters to be produced with mutually equal filter properties. Thus, the synthetic spectral distribution characteristic obtainable by the present configuration is deficient in uniformity and repeatability.
(3) To heighten the focusing efficiency, the apparatus necessitates use of large focusing mirrors or lenses and large filters. As a multilayer interference filter gains in size, it becomes extremely difficult to equalize filter characteristic in the central part and the peripheral part of the filter. Thus, the apparatus obtains as high synthetic spectral distribution as designed only with extreme difficulty.